Completed in 1771 and reworked in 1783, the San Cristobal Fort was El Morro's partner in the city of Old San Juan's defense.As if its size and height were not sufficient to intimidate enemies, its intricate modular design was sure to fool them. A strategic masterpiece, it features five independent units, each connected by moat and tunnel; each fully self-sufficient should the others fall. Open daily, tours at 10am & 2pm. San Cristobal is a World Heritage and National Historic Site.